<!doctype html>
<html>
<head>
<meta charset="utf-8">
<title>无固定width/height容器内的绝对定位元素拉伸</title>
<style>
body {
    background-color: #ddd;
}
img {
	vertical-align: bottom;
}
.container { 
	display: inline-block;
	position: relative;
}
.cover { 
	position: absolute; 
	left: 0; top: 0; right: 0; bottom: 0;
	background-color: #fff;
	/**
	 * [CSS3 opacity 属性](http://www.w3school.com.cn/cssref/pr_opacity.asp)
	 */
	opacity: .5; filter: alpha(opacity=50);
}
</style>
</head>

<body>
<span class="container">
	<i class="cover"></i>
	<img src="http://img.mukewang.com/547c34cf000171a002560191.jpg" width="512" height="381">
</span>
<span class="container">
	<i class="cover"></i>
	<img src="http://img.mukewang.com/547c34c9000171a002560191.jpg" width="256" height="191">
</span>
</body>
</html>